Our long term goal is to develop an integrated model which will define the salient differences between energy metabolism regulation in slow and fast heart-rate hearts. An understanding of these differences will allow a clearer understanding of the functions in situ of regulatory factors such as the mitochondrial ATPase inhibitor protein (IF1) in human and in other slow heart-rate hearts. The proposed research focuses on both normal regulatory and pathophysiological functions of IF1 in situ in normally functioning hearts and in ischemic cardiac muscle, respectively. Our research plan takes advantage of a species comparative approach to the elucidation of the roles played by IF1, comparing its function or lack thereof in the hearts of both slow and fast heart-rate species. While IF1 effectively slows ATP hydrolysis by the undriven mitochondrial ATP synthase during ischemia in slow heart-rate hearts, it has very little inhibitory action in fast heart-rate hearts. The planned work will address the mechanisms underlying this difference in the regulatory behavior of If1 in slow and fast heart-rate hearts. It will also address the different regulatory characteristics of two other functionally related systems, the mitochondrial phosphate carrier and phosphofructokinase (PFK) in slow and fast heart-rate hearts. pH-dependent, Pi-carrier-mediated Pi/H+ symport is primarily responsible for the drop in mitochondrial matrix pH during ischemia required for the inhibition of the mitochondrial ATPase by IF1, but only in slow heart-rate hearts. PFK stimulation mediates the large increase in glycolytic flux occurring at the onset of ischemia, an increase which is approximately five time larger in fast than in slow heart-rate hearts. The first aim of the proposed work is to determine the contribution to the net rate of ATP synthesis made by the back reaction rate of the ATP synthase in intact slow versus fast heart-rate heart mitochondria; IF1 binding regulates primarily only the back reaction rate. Our second aim is to characterize factors responsible for the lack of IF1 binding observed in fast heart-rate heart mitochondria. Our third aim is to quantitate the amounts of inhibitor present in aging and failing human heart mitochondria to see if there may be an inhibitor deficit making them intrinsically more susceptible to ischemia-mediated damage. Our fourth aim is to further characterize regulatory differences between the Pi carrier in intact slow versus fast heart-rate heart mitochondria. Our fifth and last aim is to further characterize regulatory differences between the phosphofructokinase present in slow versus fast heart-rate hearts.